1. Field of the Invention
THIS INVENTION relates to a method of, and apparatus for, purging loose magnetite from boilers. The invention is particularly suitable for, but not limited to, removing exfoliated magnetite from the bends and inner bores of chromium stainless steel boiler tubes.
2. Prior Art
Chromium stainless steel superheater and reheater tubes, which operate in a high steam temperature environment for extended periods, develop a two-part oxide layer on the inner bore of the tubes. The oxide, which is given the general name xe2x80x9cmagnetitexe2x80x9d, is characterised by two distinct phasesxe2x80x94an outer layer (closest to the tube centre) which is iron rich, and an inner layer which is chrome rich. The two layers have very different coefficients of expansion. The process of cooling out the boiler induces large stresses between the two oxides. At metal temperatures of approximately 90xc2x0-150xc2x0 C., the outer layer of magnetite oxide tends to delaminate from the tightly adhering inner layer and parent metal.
Exfoliation of this outer layer and a small amount of inner layer causes magnetite to fall and partially or totally block the bottom of vertical or pendant superheaters and reheaters. Once a total blockage occurs in a tube, the steam flow paths being established in a boiler as it builds steam pressure bypass this blocked tube (due to small differential pressures within parallel paths of the superheater/reheater pendants). The lack of cooling steam causes overheating of this blocked tube leg and results in failure of the tube through short term overheating. The time and point of failure is often not detected.
Rupture of a single tube results in that tube moving violently amongst neighbouring tubes. This permits other tubes to become damaged and potentially rupture. Steam impingement on nearby tube walls becomes another mechanism of failure. Eventually, so much steam will be lost to the gas path side of the boiler that a gas side pressure excursion will remove the boiler from service, or the boiler feed pumps will not maintain the condensate feed. Several weeks may be required to repair the damage from several hours of damage caused by a ruptured tube.
Callide, Tarong and Stanwell Power Stations in the State of Queensland, Australia, have superheaters that are vertical pendants made of 321 stainless steel, and operate at high metal temperatures (believed to be 580xc2x0-640xc2x0 C.) for extended periods. The combination causes a large amount of magnetite exfoliation to occur during boiler cool downs.
To ensure that a tube does not rupture on the boiler""s return to service, the traditional practice is to:
(a) allow the boiler to cool out using the draft fan groups;
(b) build a scaffold access to the superheater bends;
(c) x-ray all superheater loops;
(d) cut and clean all tubes which have more than a 50% section area blockage;
(e) re-weld all cut tubes and check the weld quality with x-rays; and
(f) remove the scaffold and return the boiler to service.
There still remains some risk that further falls of magnetite and possible tube blockage will occur in the loops after the initial x-rays and scope of work have been completed. This phenomenon can possibly occur due to high stresses continuing to undergo relaxation in the outer layer magnetite, the initially wet tube drying and the magnetite flakes losing their adhesion sites on the tube bore, and the action of pumping water into the superheater loops for a boiler pressure test. Case studies of Japanese boiler plants show the need to x-ray, cut and clean tubes up to three times in the same boiler shutdown.
Options suggested to stop or significantly reduce the effects of magnetite exfoliation include:
(a) replace the stainless steel boiler tube with material that exhibits a lower tendency to produce exfoliating magnetite. Retrofitting a 350MW boiler with new tube is estimated to cost as least $(AU)5M in materials and labour. A significant additional cost may be the loss of availability of the boiler;
(b) operate the boiler at a lower steam temperature (eg, below 530xc2x0 C. main steam temperature). The rate of formation of magnetite at this temperature is very much slower. The trade-off is the resulting efficiency loss;
(c) cycle the boiler so that only small amounts of magnetite exfoliation occur between boiler cool downs. This is a very costly exercise due to the significant start-up fuel costs and life expended on metal components. (The boiler materials have a finite life due to stress cycles induced by the cooling down and heating up process);
(d) perform a chemical clean on the superheaters so as to strip off the outer layer of magnetite and leave intact the inner layer to act as an impediment to the further migration of iron and the resulting formation of the outer layer of magnetite. Performing a chemical clean costs approximately $(AU)300,000 and requires at least fifteen lost generation days. Callide and Tarong units have undergone chemical cleans on the superheaters. Magnetite shredding after these chemical cleans at Callide were greater in quantity than that seen prior to the chemical cleans. Doubt exists as to the effectiveness of this method of controlling exfoliating magnetite;
(e) create a flow path for the magnetite to be expelled from the superheater loops. Devising a flow path exit for exfoliating magnetite is attractive due to its minimal once off capital cost, low technology, simplicity, speed of performance and repeatability.
One prior art suggestion proposed to generate a flow path for exfoliated magnetite is by xe2x80x9cbackwashingxe2x80x9d the superheater loops using water filled loops and compressed air to drive water and magnetite from the loops. This process has the disadvantage that the exfoliated magnetite is distributed upstream of the superheater loops, and is not removed from the boiler. It is suspected that when the boiler is recommissioned, the magnetite distributed in the boiler is not completely removed during the initial steam purges and can later cause impingement damage to the turbine components and control valves.
The prior art suggestion referred above utilises a reservoir for the compressed air/steam, introduces the compressed media to the water/magnetite solution and drives the water/magnetite slug from the superheater loops. The water/magnetite slug is initially at atmospheric pressure. Upon opening the compressed media reservoir, the water interface nearest the reservoir experiences a sudden pressure rise. This transient wave propagates through the water to the magnetite slug. The slug is compressed and radial frictional forces are significantly increased. The removal of a magnetite slug completely filling a tube bore can only happen if the pressure wave can overcome the frictional resistive forces. A critical length of magnetite slug exists whereby a nominal pressure transient will not move the slug of magnetite. A boiler tube failure at Tarong Power Station (Unit 2, February 1999), soon after a boiler restart, has been attributed to the inability of the prior art suggestion""s water/air purging technique.
The cooling down of boilers, for maintenance/repairs is another major problem.
Large capacity, high-pressure boilers found in power generation plants have numerous headers to mix and redirect the steam path. Additionally, these boilers can have one or more drums or separators. These vessels are typically made of carbon steel or alloy materials and have 40 mm-140 mm thick walls. This produces a high metal volume to surface ratio that makes cooling of the vessels difficult.
Forced outages or planned outages (shut downs) occur on boilers to enable rectification work to be performed or statutory inspections to be fulfilled. Some of this work is conducted on, in, or near the above vessels. It is typical to require a waiting time of 4-8 days for thick walled vessels to be cool enough for personnel to have skin contact. The 350 MW units at Callide Power Station, Queensland, Australia, require 4-6 days before the tertiary superheater header metal temperatures fall below 100xc2x0 C. when employing a cooling mode of natural convection assisted by boiler fan groups.
The cooling time is lengthened if the units are not fired down to very low pressures, the fan groups on the gas side are not utilised to cool the superheater/reheater pendants, waterwalls and roof tubes, or large amounts of ash collect in the heater vestibule and insulate the lagged headers. The cost of waiting several days for drums and headers to cool is significant. A reduction in this waiting time through the use of a controlled forced cooling method for these vessels would be of value to boiler owners.
It is an object of the present invention to provide a method to significantly reduce the amount of magnetite initially lodging in the boiler tube bends during the cool down process.
It is a preferred object of the present invention to effectively remove all magnetite slugs from boiler tube bends in the event that a magnetite blockage exists. The method described as part of this invention is superior to the prior art suggestion in that the advocated method causes a significant decrease in the initial frictional resistive forces experienced by the removal of a magnetite slug by the prior art suggestion. The method hereinafter described will thus permit a longer critical length of magnetite slug to be removed than by the prior art suggestion. Additionally, the present method overcomes some deficiencies of the known water/compressed air method. The present method will be shown to be a preferred alternative for exfoliated magnetite removal in superheaters or reheaters.
It is a further preferred object of the present invention to provide a method of purging exfoliated magnetite from boiler superheater loop(s) which is relatively simple, inexpensive and be performed within a minimal amount of time.
It is a still preferred object to provide a method where the magnetite is expelled from the boiler.
It is a still further preferred object to provide a dry gas flow in the tube bore during the cool down of the boilerxe2x80x94this dry gas should preferably be flowing at a rate above the terminal velocity of the magnetite flakes so that the flakes will be removed from the boiler as they exfoliate.
It is a still further preferred object to provide a method to rapidly and controllably cool the boiler to ambient temperatures using forced convection.
It is a still further preferred object to provide a method where the rate at which the cooling medium flows through the boiler is controlled so that the temperature gradient of boiler metal does not exceed preset limits.
It is a still further preferred object to provide a method where magnetite slugs are expelled from boiler tube loops by releasing a compressed gas such as air, nitrogen, dry steam, oxygen or carbon dioxide.
It is a still further preferred object to provide a method where the boiler superheater tube bores are left in a dry state to prohibit flash rusting or similar corrosion states. Additionally, exfoliated magnetite flakes have a weaker bonding force to the tube bore in the dry state. Removal of the flakes can be accomplished more easily in the dry state rather than wet.
It is a still further preferred object to purge all magnetite containing superheater or reheater tube paths simultaneously.
It is a still further preferred object to provide apparatus for the method using equipment that is readily available.
Other preferred objects will become apparent from the following description.
The present invention, in different aspects, broadly encompasses:
(a) rapidly, but controllably force, cooling of all boiler metal to ambient temperatures to enable early access to the boiler for maintenance work;
(b) avoiding superheater/reheater tube blockages caused by exfoliated magnetite through the means of introducing a gas flow in the boiler tubes to carry the exfoliating material out of the boiler. This process of removal occurs during the cool down of the boiler metal and as the magnetite flake leaves the exfoliation site;
(c) remove superheater/reheater tube blockages caused by exfoliated magnetite in the event that they do form. This aspect of the invention uses a compressible gas that has been stored under pressure within the boiler tubes containing the exfoliated magnetite. The gas is suddenly released, preferably from a point near the site of the magnetite tube blockages. The sudden pressure drop causes the magnetite slug to be expelled from the boiler tube loops. Several iterations of the gas pressurisation/depressurisation may be required to remove long slugs of magnetite or slugs of magnetite in series.
In one aspect the present invention resides in a method of controllable force cooling a boiler and removing exfoliated magnetite from at least one boiler heater tube in the boiler, the method including the steps of:
after the boiler is fired down, depressurised and drained of water, and while the temperature(s) of the boiler tube(s) exceeds the temperature at which magnetite exfoliates, introducing a dry gas flow into the boiler water/steam side of the boiler to (i) initially remove wet steam and prohibit condensation collecting in superheater loops in the boiler heater tube(s), and (ii) then promote the dry gas flow in the boiler heater tube(s) where magnetite exfoliation exists, where the flow velocity of the dry gas exceeds the terminal velocity of flakes of the magnetite exfoliated so as to cause the flakes to be expelled from the boiler.
Preferably the boiler tube temperature is above 150xc2x0 C.
Preferably the flow velocity of the dry gas exceeds 4.5 m/s. (meters/seconds).
Preferably the dry gas flow is maintained, in step (ii), until the boiler tube(s) temperature(s) are below the temperature at which magnetite exfoliation occurs. Preferably the boiler tube(s) temperature(s) are below 50xc2x0 C.
Preferably the dry gas flow is at high pressure to promote surface heat transfer between the wall(s) of the boiler tube(s) and the dry gas.
Preferably the dry gas is introduced into the coldest portions of the boiler tube(s) to minimise thermal stress.
Preferably, the dry gas is air, nitrogen, dry steam, oxygen, carbon dioxide or two or more of these in a mixture.
In a second aspect the present invention resides in a method for removing magnetite blockage(s) from at least one boiler tube in a boiler, including the steps of;
a) operably connecting a quick opening valve to the boiler tube(s);
b) slowly pressurising the boiler tube(s) with dry gas enabling the dry gas to be entrained between flakes of the magnetite during the pressurisation phase; and
c) rapidly opening the valve to cause a negative gas pressure transient to propagate back to the magnetite blockage(s), to cause the magnetite blockage(s) to be fluidised and be drawn out the boiler tube(s) by flow of the dry gas out the valve.
Steps (b) and (c) may be repeated one or more times to remove all the magnetite.
Preferably the boiler tube(s) are pressurised to 650-1200 KPa. The dry gas may be initially compressed to eg, 650 KPa and be further compressed by water pumped through boiler feed pump(s).
Preferably in step (c), the valve opening time is no longer than 0.5 seconds.
In third and fourth aspects, the present invention resides in the apparatus for effecting the methods of the first and second aspects, respectively.